Integrated circuit having chemically modified spacer surface

ABSTRACT

A method of fabricating an integrated circuit includes depositing a first dielectric material onto a semiconductor surface of a substrate having a gate stack thereon including a gate electrode on a gate dielectric. The first dielectric material is etched to form sidewall spacers on sidewalls of the gate stack. A top surface of the first dielectric material is chemically converted to a second dielectric material by adding at least one element to provide surface converted sidewall spacers. The second dielectric material is chemically bonded across a transition region to the first dielectric material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 13/427,062 filedMar. 22, 2012, and claims the benefit of Provisional Application Ser.No. 61/463,308, entitled “Surface modification to optimize wet etchresistance” filed Mar. 28, 2011, which is herein incorporated byreference in its entirety.

FIELD

Disclosed embodiments relate to semiconductor processing and integratedcircuit (IC) devices therefrom which include metal-oxide-semiconductor(MOS) transistors, including MOS transistors having multi-layer sidewallspacers.

BACKGROUND

While processing semiconductor wafers it is often advantageous todeposit or form films that can later act as etch stop layers whensubsequently deposited or formed films are removed. However, if the filmdoes not have sufficient etch resistance during later processing, suchfilms can be inadvertently removed.

One example of inadvertent removal involves thin silicon nitridesidewall (or offset) spacers for MOS transistors. Thin silicon nitridesidewall spacers are commonly used as an implant mask to provide a spacebetween the lightly doped drain (LDD) implants into the semiconductorsurface and the gate stack. A typical process flow has a first spacerlayer that acts first as an offset spacer, then as anunderlayer/etch-stop while additional films, such as disposable secondsidewall spacer comprising SiGe, is deposited on top, used, which isthen subsequently removed. In one process flow hot phosphoric acid (HPA)is used to remove the second sidewall spacer. However, even siliconnitride spacers formed from bis-tertiarybutylamino-silane (BTBAS) andammonia reagents, where BTBAS-based silicon nitride is known to be themost wet etch resistant silicon nitride film to HPA, are not alwayscapable of stopping the HPA etch when the disposable SiGe secondsidewall spacer is removed. In particular, if the silicon nitridesidewall spacer has been exposed to reducing chemistries, such asplasmas containing H₂ or N₂, etch stop characteristics can be lostresulting in inadvertent removal of the silicon nitride offset sidewallspacer, and as a result subsequent shorting between the gate and sourceand/or drain, such as due to a subsequently deposited silicide ion thegate, source and drain. Moreover, as semiconductor devices are shrunk insize, and the distance between the top of the gate stack and the topsurface of the source/drain regions is reduced, the probability ofelectrical shorts due to the silicide forming on the sidewalls of thegate stack increases.

SUMMARY

Disclosed embodiments describe solutions to the above-describedinadvertent removal of thin sidewall spacers formetal-oxide-semiconductor (MOS) transistors that use multi-layersidewall spacers. By chemically converting the top surface of a firstsidewall spacer comprising a first material by adding at least oneelement to form a second dielectric material, the second material cansubstantially increase the etch resistance compared to the first spacermaterial. As a result, the subsequent removal of a disposable secondspacer on the first spacer will not remove the first spacer since thesecond dielectric material can act as an etch stop, or at least providesome etch protection for the first dielectric material of the firstspacer.

One disclosed embodiment comprises a method of fabricating an integratedcircuit that includes depositing a first dielectric material onto asemiconductor surface of a substrate having a gate stack thereonincluding a gate electrode on a gate dielectric. The first dielectricmaterial is etched, such as using RIE, to form sidewall spacers onsidewalls of the gate stack. A top surface of the first dielectricmaterial is chemically converted to a second dielectric material byadding at least one element to provide surface converted sidewallspacers. The second dielectric material is chemically bonded across atransition region to the first dielectric material.

Following forming the surface converted sidewall spacers, ion implantingcan follow to form lightly doped drains (LDDs) in the semiconductorsurface lateral to the gate stack. Second spacers are then formed on thesurface converted sidewall spacers. Sources and drains are then formedlateral to the gate stack. Ion implanting can be used to form sourcesand drains in the semiconductor surface lateral to the gate stack afterforming the second spacers. Alternatively, the second sidewall spacerscan be used for a SiGe S/D process (e.g., where recesses are formedtypically in the PMOS region and replaced with SiGe). The second spacerscan then be selective removed after the source/drain formation. Thesurface of chemically converted layer remains intact after the selectiveetching, as does the first dielectric material protected by the surfaceconverted layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIG. 1 is a flow chart that shows steps in an example method forfabricating an integrated circuit (IC) device having MOS transistorsthat include surface converted sidewall spacers, according to an exampleembodiment.

FIG. 2A-2F are cross-sectional diagrams depicting processing progressionfor an example method of forming an IC device having MOS transistorsthat include surface converted sidewall spacers, according to an exampleembodiment, while FIG. 2G shows the resulting spacer structure after aknown spacer process showing the results from the inadvertent removal ofthe nitride offset spacer.

FIG. 3 is a cross sectional view of a portion of an IC device includingMOS transistors having sidewall spacers comprising a second dielectricmaterial on a first dielectric material, wherein the second dielectricmaterial is chemically bonded across a transition region to the firstdielectric material, according to an example embodiment.

FIG. 4 shows the composition as a function of thickness for an examplesurface converted sidewall spacer, including a highly simplifieddepiction of the chemical bonding provided across the thickness of thesurface converted sidewall spacer, according to an example embodiment.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

FIG. 1 is a flow chart that shows steps in an example method 100 forfabricating an IC device having MOS transistors that include surfaceconverted sidewall spacers, according to an example embodiment. Step 101comprises depositing a first dielectric material onto a semiconductorsurface of a substrate having a gate stack thereon comprising a gateelectrode on a gate dielectric. Step 102 comprises etching the firstdielectric material to form sidewall spacers on sidewalls of the gatestack, such as using RIE.

Step 103 comprises chemically converting a top surface of the firstdielectric material to a second dielectric material by adding at leastone element to provide surface converted sidewall spacers. The seconddielectric material is chemically bonded across a transition region tothe first dielectric material. The chemically converted top surface ofthe sidewall spacer becomes an etch stop by adding at least one elementto form a second dielectric material, that substantially increases thewet etch resistance of the film as compared to the unconverted firstdielectric material, such as to a hot phosphoric acid (HPA) etch. In oneembodiment the added element is carbon. In another embodiment bothcarbon and oxygen are added.

In one specific example, the first dielectric material comprisesBTBAS-derived silicon nitride, and carbon is added to the top surface ofthe silicon nitride forming a thin layer, typically 10 to 20 Angstromsthick, of a second dielectric material comprising a silicon carbide(SiC), silicon carbonitride (SiCN) and/or silicon oxy-carbonitride(SiOCN) film. This can be accomplished by exposing a BTBAS siliconnitride film that was previously used as a gate stack sidewall to a flowof 30 to 3000 sccm of ethylene, acetylene, or similar hydrocarbon gas ata temperature generally between 300 and 800° C., and pressure betweenabout 0.1 and 10 Torr, for 15-600 seconds or longer prior to depositinga subsequent disposable spacer film. In experiments performed, SiC,SiCN, or SiOCN were formed, which were all found to be are largelyimpervious to HPA etch at temperatures below 215° C. Since HPA isgenerally used at temperatures between 120 and 180° C., the underlyingsilicon nitride sidewall spacer is protected by the second dielectricmaterial.

Besides clear process differences, the relationship of the seconddielectric material to the first dielectric material for disclosedsurface converted sidewall spacers being chemically bonded together isdistinct from known arrangements resulting from the vapor deposition(e.g., chemical vapor deposition) of a second dielectric material on afirst dielectric material, where the second dielectric material becomesattached to the first dielectric material by comparatively weak Vanderwalls forces. Moreover, inherently due to the disclosed chemicalconversion process, the area of the second dielectric material matchesthe area of the first dielectric material. In contrast, for a knownarrangements resulting from the vapor deposition of a second dielectricmaterial on a first dielectric material, the area of the seconddielectric material will be different as compared to the area of thefirst dielectric material due to the etching process required for spacerformation.

Step 104 comprises ion implanting to form lightly doped drains (LDDs) inthe semiconductor surface lateral to the gate stack. For a CMOS processthe PMOS transistors and NMOS transistors generally each receiveseparate LDD implants. Step 105 comprises forming second spacers on thesurface converted sidewall spacers. Step 106 comprises forming sourcesand drains lateral to the gate stack. Ion implanting can be used to formsources and drains in the semiconductor surface lateral to the gatestack after forming the second spacers. For a typical CMOS process thePMOS transistors and NMOS transistors each receive separate source/drainimplants. However, alternatively, the second sidewall spacers can alsobe used for a SiGe S/D process (e.g., where recesses are formedtypically in the PMOS region and replaced with SiGe). Step 107 comprisesselectively removing the second spacers after the source/drain formation(step 106). The surface of chemically converted layer remains intactafter the selective etching, as does the first dielectric materialprotected by the surface converted layer.

FIG. 2A-2F are cross-sectional diagrams showing processing progressionfor an example method of fabricating an IC device having surfaceconverted sidewall spacers, according to an example embodiment, whileFIG. 2G shows the resulting spacer structure after a known spacerprocess showing inadvertent removal of the sidewall spacer. FIG. 2Ashows a gate stack comprising a gate electrode 211 on a gate dielectric212 before any sidewall spacer is formed on a substrate 305. Substrate305 can comprise any substrate material, such as silicon,silicon-germanium, as well as II-VI and III-V substrates, as well as SOIsubstrates. The gate electrode 211 can comprise polysilicon, or avariety of other gate electrode materials. The gate dielectric 212 cancomprise a variety of gate dielectrics, including optional high-kdielectrics defined hereon as having k>3.9, typically a k>7. In oneparticular embodiment, the high-k dielectric comprises siliconoxynitride.

FIG. 2B shows the gate stack after a sidewall spacer (e.g., a nitrideoffset spacer) 215 is formed, such as a silicon nitride offset spacer bya RIE process. FIG. 2C shows the results after an ion implantationprocess, such as LDD ion implantation to form LDD regions 225, thatutilized implant blocking provided by the sidewall spacer 215. FIG. 2Dshows the resulting structure after disclosed chemical surfaceconversion step comprising flowing a hydrocarbon gas that forms thesurface converted layer 216 shown. FIG. 2E shows the gate stack 211/212after a subsequent disposable second spacer 235 is formed, such as bychemical deposition followed by

RIE. For a typical CMOS process the PMOS transistors and NMOStransistors each then receive separate source/drain implants.

The disposable second spacer 235 is then selectively removed aftersource/drain formation. FIG. 2F shows the gate stack 212/211 after thedisposable second spacer 235 has been selectively removed, such as by ahot (e.g., 120 to 180° C.) HPA etch. Note the surface converted layer216 remains intact after the etch, as does the sidewall spacer 215protected by the surface converted layer 216. Without a disclosedsurface converted layer, the sidewall spacer 215, such as it comprisessilicon nitride, is subject to removal using the process used to removethe disposable second spacer 235. FIG. 2G shows the resulting spacerstructure after a known spacer process showing the results afterinadvertent complete removal of the sidewall spacer 215.

FIG. 3 is a cross sectional view of a portion of an IC device 300 (e.g.,a semiconductor die) including MOS transistors having surface convertedsidewall spacers comprising a second dielectric material on a firstdielectric material, wherein the second dielectric material ischemically bonded across a transition region to the first dielectricmaterial, according to an example embodiment. Back end of the line(BEOL) metallization is not shown for simplicity. IC 300 includes asubstrate 305, such as a p-type silicon or p-type silicon-germaniumsubstrate, having a semiconductor surface 306. Optional trench isolation308 is shown, such as shallow trench isolation (STI). An n-channel MOS(NMOS) transistor 310 is shown, along with a p-channel MOS (PMOS)transistor 320 that is within an n-well 307.

NMOS transistor 310 includes a gate stack including a gate electrode 311on a gate dielectric 312 having sidewall spacers on sidewalls of thegate stack. The sidewall spacers comprise a second dielectric material315 a on a first dielectric material 315 b, wherein the seconddielectric material 315 a is chemically bonded across a transitionregion 315 c to the first dielectric material 315 b. The seconddielectric material 315 a comprises carbon and the first dielectricmaterial does not comprise carbon, wherein “not comprising carbon” asused herein refers to a wt. % of C<3%.

NMOS transistor 310 includes source 321 and drain 322 regions lateral tothe sidewall spacers, and include lightly doped extensions 321 a and 322a. A silicide layer 316 is shown on the gate electrode 311 and thesource 321 and drain 322.

Similarly, PMOS transistor 320 includes a gate stack including a gateelectrode 331 on a gate dielectric 332 (which can be the same materialas gate dielectric 312 under gate electrode 311) having sidewall spacerson sidewalls of the gate stack, comprising the second dielectricmaterial 315 a on a first dielectric material 315 b, wherein the seconddielectric material 315 a is chemically bonded across a transitionregion 315 c to the first dielectric material 315 b. The seconddielectric material 315 a comprises carbon and the first dielectricmaterial does not comprise carbon. PMOS transistor 320 includes source341 and drain 342 regions lateral to the sidewall spacers, and includelightly doped extensions 341 a and 342 a. Silicide layer 316 is shown onthe gate electrode 331 and on the source 341 and drain 342.

The total thickness of the sidewall spacer 315 a/315 c/315 b at itswidest point at its base is generally ≦100 Angstroms, such as 40 to 70Angstroms thick. For example, in one particular embodiment seconddielectric material 315 a is about 5 to 10 angstroms thick, transitionregion 315 c is 15 to 25 Angstroms thick, and the first dielectricmaterial 315 b is 20 to 30 Angstroms thick.

FIG. 4 shows the composition as a function of thickness for an examplesurface converted sidewall spacer 400, including a highly simplifieddepiction of the chemical bonding provided across the thickness of thesurface converted sidewall spacer 400, according to an exampleembodiment. The surface converted sidewall spacer 400 includes anon-constant chemical composition profile across its thicknesscomprising a first dielectric material 315 b on the sidewall of a gatestack material and a chemically converted top (outer) surface comprisinga second dielectric material 315 a chemically bonded across a transitionregion 315 c to the first dielectric material 315 b. In the embodimentshown the first dielectric material 315 b comprises silicon nitride(roughly Si₃N₄), the second dielectric material 315 a comprises siliconcarbide (SiC), and the transition region 315 c includes a materialcomprising Si, N and C, where the C content decreases and the N contentincreases as the distance to the second dielectric material 315 a /gatestack is reduced.

Disclosed semiconductor die may include various elements therein and/orlayers thereon, including barrier layers, dielectric layers, devicestructures, active elements and passive elements including sourceregions, drain regions, bit lines, bases, emitters, collectors,conductive lines, conductive vias, etc. Moreover, the semiconductor diecan be formed from a variety of processes including bipolar, CMOS,BiCMOS and MEMS.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure.

We claim:
 1. An integrated circuit (IC), comprising: a substrate havinga semiconductor surface, at least one metal-oxide-semiconductor (MOS)transistor on said semiconductor surface, said MOS transistorcomprising: gate stack including a gate electrode on a gate dielectrichaving sidewall spacers on sidewalls of said gate stack, said sidewallspacers comprising a second dielectric material on a first dielectricmaterial, and source and drain regions lateral to said sidewall spacers,wherein said second dielectric material comprises carbon and said firstdielectric material does not comprise carbon, and wherein said seconddielectric material is chemically bonded across a transition region tosaid first dielectric material.
 2. The IC of claim 1, wherein said firstdielectric material comprises silicon nitride and said second dielectricmaterial comprises (SiC), silicon carbonitride (SiCN) or siliconoxy-carbonitride (SiOCN).
 3. The IC of claim 1, wherein an area of saidsecond dielectric material matches an area of said first dielectricmaterial.
 4. The IC of claim 1, wherein a total thickness of saidsidewall spacers is ≦100 Angstroms.